1. Field of the Invention
The present invention relates to a wafer polishing device and, more particularly, to a wafer polishing device for removing a surface unevenness of a semiconductor substrate.
2. Description of the Related Art
FIG. 4 shows a conventional wafer polishing device. The polishing device includes a rotatable polishing table 1, a polishing pad 4 provided on the polishing table 1 and a carrier head 8 for supporting a semiconductor substrate 2, that is, a wafer 2, to be polished. The polishing apparatus further includes a pressing mechanism 9 for pressing the wafer 2 together with the carrier head 8 to the polishing pad 4 and a spindle 10 for rotating the wafer 2 with the carrier head 8 on the polishing pad 4. A polishing slurry 5 is supplied onto a surface of the polishing pad 4 by means of a pump, etc from a polishing slurry nozzle 11.
The wafer 2 is pressed by the pressing mechanism 9 to the rotating polishing pad 4 through the spindle 10 and the carrier head 8 so that the wafer 2 is polished by the polishing pad 4. For example, the wafer contains a plurality of semiconductor layers, which are polished by CMP (chemical mechanical polishing) method to flatten a surface of the wafer including them.
The wafer polishing device further includes a retainer ring 3 for holding the wafer on the polishing pad 4. The retainer ring 3 may take in the form of non-contact type retainer shown in FIG. 5 or in the form of contact type retainer 3 shown in FIG. 6. The retainer ring 3 shown in FIG. 5 is used only for retaining the wafer 2 in a predetermined position on the polishing pad 4 without contacting with a surface of the polishing pad 4 so that the slurry 5 can be easily supplied into between the polishing pad 4 and the wafer 2. However, when such retainer ring is used, load is concentrated between an edge portion of the wafer 2 and the polishing pad 4. Therefore, the edge portion of the wafer 2 is polished at higher speed than the surface except the edge portion of the wafer 2 so that the peripheral portion of the wafer 2 is abnormally polished as shown in FIG. 7. Therefore, the flatness of the outer peripheral portion of the wafer 2 is degraded as shown in FIG. 9, resulting in the yield of wafer being reduced.
In order to solve the problem caused by the non-contact type retainer ring shown in FIG. 5, the retainer ring 3 shown in FIG. 6 is used. This retainer ring 3 is always in contact with the surface of the polishing pad 4. According to the retainer ring 3 shown in FIG. 6, the load to be applied concentrically between the polishing pad 4 and the wafer 2 is dispersed and the load point is moved outward of the retainer ring 3. Therefore, the polishing speed profile at the edge portion of the wafer is improved as shown in FIG. 8.
However, there is a problem when the retainer ring 3 shown in FIG. 6 is used. That is, in the wafer polishing device using the retainer ring 3 of the contact type, the supply of slurry into between the wafer 2 and the polishing pad 4 becomes short since the polishing pad 4 is in contact with the retainer ring 3. Further, since the slurry 5 already supplied tends to stay within the retainer ring 3, the supply of new slurry into the retainer ring 3 is blocked, causing the polishing speed for the surface of the wafer 2 to be varied. Therefore, the polishing profile in the edge portion of the wafer is varied.